On the origin of gypsum in the Mars north polar region

[1] We describe the distribution and concentration of the largest Martian gypsum deposit discovered to date by the Mars Express OMEGA (Observatoire pour le Mineralogie, l'Eau, les Glaces et l'Activite ) imaging spectrometer, its relationship to the late Amazonian-aged north polar dunes in which it is found, and its likely origin. Gypsum has not been discovered anywhere within the north polar region outside of the Olympia Undae dune sea. In the areas of highest gypsum a concentration, 35% pure gypsum grains of a few tens of micrometers in size, mixed with 65% millimeter-sized gypsum grains containing, dark, spectrally featureless inclusions best fit the OMEGA observations. The gypsum-rich dunes contain no significant average albedo, temperature, or morphological anomalies. We propose that water emanating from nearby channels, carved during melting of the polar layered deposits, infiltrated the eastern end of the polar dune sea, percolating through the dunes. Deposits of gypsum resulted from a combination of direct, in situ alteration of sulfide- and high-calcium-pyroxene-bearing dunes and from formation of evaporitic gypsum crystals in the pore spaces of these dunes. This gypsum deposit formed in a unique local environment and is disconnected from sulfate-forming events elsewhere on Mars which are thought to have occurred much earlier, during the late Noachian and Hesperian, by various means. Sulfates have not been discovered in any other collection of dunes on Mars.